Hi,
Are you in HEXADECIMAL on your Prime ? (look at your home settings)
You have to enter the color like that (for HEXA): RECT_P(#RRGGBBh),
with RR,GG,RR from 00h to FFh.
# is a prefix which tells that you are using a binary integer.
h is the suffix of the base used for the binary integer (h for hexadecimal, d for decimal, b for binary, o for octal).